The present invention is directed to new and useful films, and especially heat shrinkable films. One distinguishing feature of a shrink film is the film's ability, upon exposure to a certain temperature, to shrink or, if restrained from shrinking, to generate shrink tension within the film.
The manufacture of shrink films, as is well known in the art, may be generally accomplished by extrusion of the resinous materials which have been heated to their flow or melting point from an extrusion die in tubular or planar form. After a post extrusion quenching to cool, the extrudate is then reheated to its orientation temperatures range. The orientation temperature range for a given film will vary with the different resinous polymers and blends thereof which comprise the film. However, the orientation temperature range may generally be stated to be above room temperature and below the melting point of the film.
The terms "oriented" or "orientation" are used herein to describe the process and resultant product characteristics obtained by stretching and immediately cooling a resinous polymeric material which has been heated to its orientation temperature range so as to revise the molecular configuration of the material by physical alignment of the molecules to improve mechanical properties of the film such as, for example, shrink tension and orientation release stress. Both of these properties may be measured in accordance with ASTM D 2838-69 (reapproved 1975). When the stretching force is applied in one direction uniaxial orientation results. When the stretching force is applied in two directions biaxial orientation results. Orientation is also herein used interchangeably with "heat shrinkability" with these terms designating a material which has been stretched and set by cooling at its stretched dimensions. An oriented (i.e., heat shrinkable) material will tend to return to its original unstretched dimensions when heated to an appropriate temperature below its melting temperature range.
The term "lap seal strength" is used herein to mean the strength of the film of the present invention when sealed to itself, primarily assisted only by the weight of the product which is being packaged. Typically, shrink tunnel heat is employed to create the lap seal. This is to be contrasted to "heat seal strength" referring typically to conventional heat seals created by the pressure of e.g. an impulse sealer. Typically, seal bar heat is employed to create the heat seal.
Returning to the basic process for manufacturing the film as discussed above, it can be seen that the film once extruded and initially quenched to cool is then reheated to its orientation temperature range and oriented. The stretching to orient may be accomplished in many ways such as, for example, by "blown bubble" techniques or "tenter framing". These terms are well known to those in the art and refer to orientation steps whereby the material is stretched in the cross or transverse direction (TD) and in the longitudinal or machine direction (MD). After being stretched, the film is rapidly cooled to quench and thus set or lock-in the oriented molecular configuration.
After locking-in the oriented molecular configuration the film may then be stored in rolls and utilized to tightly package a variety of items. In this regard, the product to be packaged its first enclosed in the heat shrinkable material by heat sealing the shrink film to itself where necessary. Thereafter, the enclosed product is subjected to elevated temperatures by, for example, passing the product through a hot air or hot water tunnel. This causes the film to shrink around the product (also inducing creation of the lap seal) to produce a tight wrapping that closely conforms to the contour of the product.
The above general outline for manufacturing films is not meant to be all inclusive since this process is well known to those in the art. For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of these patents are hereby incorporated by reference.
Many variations on the above discussed general processing theme are available to those in the art depending upon the end use for which the film is to be put and the characteristics desired to be instilled in the film. For example, the molecules of the film may be cross-linked during processing to improve the films abuse resistance and other characteristics. Cross-linking and methods for cross-linking are well known in the art. Cross-linking may be accomplished by irradiating the film or, alternatively, may be accomplished chemically through the utilization of e.g. peroxides. Another possible processing variation is the application of a fine mist of silicone spray to the interior of the freshly extruded material to improve the further processability of the material.
In packaging applications involving the application of a heat seal to close a package, a film with high shrink tension can sometimes undergo seal failure or weakening during a subsequent heat shrinking step. Where the seal is a lap seal, e.g. an overlap of film typically on the underside of a package, seal failure can result in loss, damage, or contamination of the packaged product.
The inventor has discovered that the lap seal strength of certain packaging films can be significantly improved by using a blend of ethylene propylene copolymer and polybutylene in the outer layers of such films.
Alternatively, the lap seal strength of certain packaging films can be significantly improved by using a blend of ethylene propylene copolymer and propylene butene copolymer in the outer layers of such films.
Films having outer layers comprising propylene butene copolymer are also beneficial.